1. Field of the Invention
The present invention relates to a monochromator.
2. Description of the Related Art
As a conventional monochromator to be referred to, a structure in a case where a lens is used in a monochromator of Littrow type is shown in FIGS. 5 and 6. FIG. 5 is a top view of the monochromator, and FIG. 6 is an explanatory diagram in which the structure of this monochromator is developed from left to right along the optical path, and the z-axis indicates the height direction. In these drawings, P1 designates an optical fiber; P11, a lens; P5, a diffraction grating; and P9, an output slit.
As shown in FIG. 6, since the height of the output slit P9 differs from the height of the optical fiber P1, the center line of the output light from the optical fiber P1 differs from the center line of the lens P11.
The output light from the optical fiber P1 is transmitted through the lens P11 on the lower side (or upper side) of its center and becomes parallel, and is incident upon the diffraction grating P5. The center line of the output light before it is transmitted through the lens is not aligned with the center line of the transmitted light, and the light is bent.
The transmitted light incident upon the diffraction grating P5 is reflected at a different angle according to the wavelength, and this reflected light is transmitted again through the lens P11 on the upper side (or lower side) of its center and is condensed. Then, the condensed light is emitted from the output slit P9.
With such a structure, it is possible to extract only a particular wavelength component from the output slit P9. In addition, the particular wavelength which is extracted can be changed by rotating the diffraction grating PS.
However, with the structure in which the lens is used in the conventional monochromator of Littrow type, since the center line of the incident light upon the lens P11 and the center line of the output light therefrom is offset from the center line of the lens P11, aberrations occur. For this reason, the resolving power deteriorates, and the dynamic range of the light to be measured becomes narrow, resulting in a decline in the characteristics of the monochromator.
Further, the resolving power RB in the entire monochromator including the diffraction grating P5 can be approximately expressed by the following formula: EQU RB=2d/(m.multidot.f).multidot.S.multidot.cos .beta. (1)
where d is the groove spacing of the diffraction grating P5, m is the order of diffraction of the diffraction grating P5, f is the focal length of the lens, S is the slit width of the output slit P9, and .beta. is the angle formed by the reflected light from the diffraction grating P5 and the normal to the diffraction grating P5.
In the layout of the Littrow type, however, since the reflected light from the diffraction grating P5 is at the same angle as the incident light as shown in FIG. 7, the angle .beta. between the reflected light from the diffraction grating P5 and the normal to the diffraction grating P5 cannot be set to a value more than the angle of rotation of the diffraction grating P5. For this reason, as is apparent from the formula (1), the angle .beta. becomes small, so that it is theoretically difficult to expect a high resolving power. Here, the angle of rotation of the diffraction grating P5 is the angle which is formed by, on one hand, a bisector of the incident optical axis and the reflective optical axis of the diffraction grating P5 and, on the other hand, the normal to the diffraction grating P5, and assumes a predetermined angle according to the wavelength.